To reduce the quantity of NOX and particulate matter emitted to the atmosphere during internal combustion engine operation, a number of exhaust aftertreatment devices have been developed. A need for exhaust aftertreatment systems particularly arises when diesel combustion processes are implemented. Typical aftertreatment systems for diesel engine exhaust may include one or more of a diesel particulate filter (DPF), a selective catalytic reduction (SCR) system, a hydrocarbon (HC) injector, and a diesel oxidation catalyst (DOC).
During engine operation, the DPF traps soot emitted by the engine and reduces the emission of particulate matter (PM). Over time, the DPF becomes loaded and begins to clog. Periodically, regeneration or oxidation of the trapped soot in the DPF may be required for proper operation. To regenerate the DPF, relatively high exhaust temperatures in combination with an ample amount of oxygen in the exhaust stream are needed to oxidize the soot trapped in the filter.
The DOC is typically used to generate heat useful for regenerating the soot loaded DPF. When hydrocarbons (HC) are sprayed over the DOC at or above a specific light-off temperature, the HC will oxidize. This reaction is highly exothermic and the exhaust gases are heated during light-off. The heated exhaust gases are used to regenerate the DPF.
Under many engine operating conditions, however, the exhaust gas is not hot enough to achieve a DOC light-off temperature of approximately 300° C. As such, DPF regeneration does not passively occur. Furthermore, NOX adsorbers and selective catalytic reduction systems typically require a minimum exhaust temperature to properly operate.
A burner may be provided to heat the exhaust stream upstream of the various aftertreatment devices. Known burners have successfully increased the exhaust temperature of relatively small displacement internal combustion engines for automotive use. However, other applications including diesel locomotives, stationary power plants, marine vessels and others may be equipped with relatively large diesel compression engines. The maximum exhaust mass flow rate from the larger engines may be more than ten times the maximum flow rate typically provided to the burner. While it may be possible to increase the size of the burner to account for the increased exhaust mass flow rate, the cost, weight and packaging concerns associated with this solution may be unacceptable.
Furthermore, it is oftentimes desirable to operate the large displacement engines at relatively low output levels. The minimum exhaust mass flow rate may be twenty times less than the maximum mass flow rate. Therefore, a need may exist in the art for an arrangement to reduce the emissions of the exhaust output from a large diesel engine during all phases of operation while minimally affecting the cost, weight, size and performance of the exhaust system.